WASHINGTON — About 1 in 5 drivers who were killed last year in car crashes tested positive for drugs, raising concerns about the impact of drugs on auto safety, the government reported Tuesday.

Researchers with the National Highway Traffic Safety Administration said the new data underscored a growing problem of people driving with drugs in their systems. But they cautioned that it was not clear that drugs caused the crashes and more research was needed to determine how certain drugs can hinder a person's ability to drive safely.

Drugs were reported in nearly 4,000 drivers who were killed in 2009, or 18 percent of the nearly 22,000 drivers killed last year. In 2005, drugs were found in the systems of 13 percent of the more than 27,000 drivers killed in car accidents.

Researchers said the numbers could be higher because only about 3 in 5 drivers who were killed in car crashes were tested for drugs after the crash and testing varied from state to state. Among all the drivers who were killed in 2009 and later tested by authorities for drugs, about one-third had drugs in their systems.

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"Drugged driving is as inexcusable as drunk driving or driving irresponsibly," said NHTSA administrator David Strickland.

Researchers are just beginning to understand the problem. Strickland said the data did not allow them to know whether the influence of drugs caused the fatal crashes. He said testing procedures were evolving and many states and communities test for different drugs or use different types of tests.

The tests took into account both legal and illegal drugs, including heroin, methadone, morphine, cocaine, methamphetamine, marijuana, LSD, prescription drugs and inhalants. The amount of time the drug could linger in the body varied by drug type, the researchers said, so it was unclear when the drivers had used the drugs prior to the fatal crashes.

Many drugs can affect a driver's judgment and reaction time but researchers are still trying to determine the level of drug use that can impair a driver's ability to drive safely. A blood alcohol level of 0.08 is the legal limit for all 50 states but a similar level of impairment is uncertain for many drugs.

"It's very clear that we've got a significant problem," Kerlikowske said. "We've made great progress on alcohol-impaired driving through education and enforcement. There's just no reason we won't be able to make progress in this area once we start bringing it to people's attention and we start doing the enforcement that's needed."

Some recent high-profile crashes have involved drug use by drivers.

In July 2009, a New York mother sped the wrong way for more than a mile with a minivan full of children, leading to a crash that killed her and seven others. The woman had a blood-alcohol level 2½ times the legal limit and had smoked marijuana within an hour of the crash.

In Phoenix, the driver of a dump truck struck a group of motorcycle riders in March, killing four people and injuring five others. Initial tests found the driver had methamphetamine in his system.

Kerlikowske said efforts against drugged driving could be helped by improved testing procedures and standards for detecting drug use by drivers, along with more police officers trained to detect drug use by motorists.

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